Allotropes crystalline modifications

Entropy of Methylammonium Chloride. Heat capacities for this solid in its various crystalline modifications have been determined [10] precisely down to 12 K. Some of these data are summarized in Figure 11.3. There are three crystalline forms between OK and 298K. One can calculate the entropy by integrating Equation (11.21) for each allotrope in the temperature region in which it is most... 

Silvery-white metal exhibits three crystalline modifications an orthorhombic alpha form, stable at ordinary temperatures and density 20.45 g/cm the alpha-form transforms to a tetragonal beta allotrope of density 19.36 g/cm when heated at 280°C the beta form converts to a body-centered cubic crystaUine gamma modification at 577°C, having a density 18.0 g/cm . 

Allotropic forms of sulfur. Solid sulfur exists in two crystalline modifications. Rhombic sulfur consists of S8 molecules, is stable at temperatures below 95.5°C, has a specific gravity of 1.96, and is soluble in carbon disulfide. At 95.5°C, rhombic sulfur changes slowly, with absorption of heat, into the monoclinic form. Molten rhombic sulfur consists of S8 molecules and exists as a pale-yellow, thin, and limpid liquid known as X sulfur. When the temperature is raised, X sulfur is slowly converted to dark and viscous p sulfur, which consists of Ss and S4 molecules and which is considered to be the amorphous variety of... 

Allotrope S7 is known in four crystalline modifications, one of which (5 form) is obtained by crystallization from CS2 at —78°C. It features one long bond (bond S6-S7 in Fig. 16.4.1) of 218.1 pm, which probably arises from the virtually coplanar set of atoms S4-S6-S7-S5, which leads to maximum repulsion between nonbonding lone pairs on adjacent S atoms. As a result of this weakening of the S6-S7 bond, the adjacent S4-S6 and S5-S7 bonds are strengthened (199.5 pm), and there are further alternations of bond lengths (210.2 and 205.2 pm) throughout the molecule. 

The particular advantage of diffraction analysis is that it discloses the presence of a substance as that substance actually exists in the sample, and not in terms of its constituent chemical elements. For example, if a sample contains the compound A By, the diffraction method will disclose the presence of A B as such, whereas ordinary chemical analysis would show only the presence of elements A and B. Furthermore, if the sample contained both A B, and Aj Bjy, both of these compounds would be disclosed by the diffraction method, but chemical analysis would again indicate only the presence of A and B. To consider another example, chemical analysis of a plain carbon steel reveals only the amounts of iron, carbon, manganese, etc., which the steel contains, but gives no information regarding the phases present. Is the steel in question wholly martensitic, does it contain both martensite and austenite, or is it composed only of ferrite and cementite Questions such as these can be answered by the diffraction method. Another rather obvious application of diffraction analysis is in distinguishing between different allotropic modifications of the same substance solid silica, for example, exists in one amorphous and six crystalline modifications, and the diffraction patterns of these seven forms are all different. 

Tin has three crystalline modifications or allotropes, a-tin or gray tin (diamond structure), P-tin or white tin , and y-tin the latter two are metallic with close packed structures. Tin also has several isotopes. It is used in a large number of alloys including Babbit metal, bell metal, Britannia metal, bronze, gun metal, and pewter as well as several special solders. 

Carbyne. The crystalline structure of carbyne, the new carbon allotrope, as determined from A"-ray powder diffraction data has been re-evaluated. Although previously considered as a single-phase system, excellent agreement between the experimental and theoretical data has now been obtained on the assumption that there are two hexagonal crystalline modifications of carbyne (a- and jS-carbyne) with the cell parameters shown in Table 3. Evidence is... 

In the preceding section the behavior of the catalyst at Al/ Ti 1.0 was examined. Next, lower ratios will be discussed, but first it is instructive to include some description of a-TiCla, another crystalline modification of the trichloride (Natta et al., 1961a). In combination with trialkylaluminum or dialkylalumi-num chloride the a form produces trans-l,4-poly dienes with butadiene or isoprene (Natta et al., 1959b). The reason for the difference in behavior between the /3 and a modifications has not definitely been established, but it is thought to be related to the different Ti-Ti ionic distances (Saltman, 1963). In /3-Ti-CI3 this is 2.9 A, about the same as the 1-4 carbon-carbon distance for isoprene in the cis conformation. The a-TiCls has a Ti-Ti distance of 3.54 A, more in line with the 1-4 carbon-carbon distance for isoprene in the tram conformation (3.7 A). Perhaps these atomic distances are fortuitously similar, but if one assumes two-point coordination of monomer on the surface the difference between the allotropic forms can be explained. 

Like sulphur, selenium exists in a number of allotropic forms. These include both crystalline, rhombic and monoclinic modifications... 

The metal has a silvery appearance and takes on a yellow tarnish when slightly oxidized. It is chemically reactive. A relatively large piece of plutonium is warm to the touch because of the energy given off in alpha decay. Larger pieces will produce enough heat to boil water. The metal readily dissolves in concentrated hydrochloric acid, hydroiodic acid, or perchloric acid. The metal exhibits six allotropic modifications having various crystalline structures. The densities of these vary from 16.00 to 19.86 g/cms. 

Phosphorus (like C and S) exists in many allotropic modifications which reflect the variety of ways of achieving catenation. At least five crystalline polymorphs are known and there are also several amorphous or vitreous forms (see Fig. 12.3). All forms, however, melt to give the same liquid which consists of symmetrical P4 tetrahedral molecules, P-P 225 pm. The same molecular form exists in the gas phase (P-P 221pm), but at high temperatures (above 800°C) and low pressures P4 is in equilibrium with the diatomic form P=P (189.5 pm). At atmospheric pressure, dissociation of P4 into 2P2 reaches 50% at 1800°C and dissociation of P2 into 2P reaches 50% at 2800°. 

Silvery-white, brittle metallic element crystal system-hexagonal, rhombo-hedral also, exists in two unstable allotropic forms— a yellow modification and a dark-grey lustrous amorphous powder—both of which revert to crystalline form hardness 3.0 to 3.5 Mohs density 6.697g/cm3 melting point 630.5°C boiling point 1635°C electrical resistivity 39.1 microhm-cm at 0°C magnetic susceptibifity —0.87 x 10 emu/g. 

Black tetragonal crystal exhibits two allotropic modifications—a stable alpha phase, occurring in tetragonal crystalline form (as hausmannite) and an unstable beta modification density 4.85 g/cm Moh s hardness 5.5 melts at 1,567°C insoluble in water soluble in hydrochloric acid. 

Trigonal crystalline solid or amorphous powder mineral millerite has a yellow metallic luster color varies from yellow to brownish black density 5.30 to 6.65 g/cm3 exhibits three allotropic modifications (1) the acid-soluble amorphous alpha form obtained from nickel salt solution by precipitation with ammonium sulfide, (2) the alpha form rapidly transforms to a crystalline beta form as a brown colloidal dispersion upon exposure to air, and (3) a rhombo-hedral gamma modification found native as mineral millerite, which also can be prepared artificially under certain conditions. 

Two crystalline forms exist (1) alpha allotrope a simple cubic low temperature form density 9.196 g/cm, and (2) beta modification a rhombohedral high temperature form density 9.398 g/cm ... 

On elevation of the temperature unstable allotropes become converted more rapidly than at low temperatures into the stable modification and the increased mobility of atoms at high temperatures produces a similar modification in the rapidly cooled conglomerates or amorphorus substances this conversion into the crystalline accompanied by a grain growth or a conversion of the micro-crystalline into the macrocrystalline state is termed sintering the preliminary stage in annealing. 

Allotropic forms of carbon. In the solid state, the element carbon exists in three different allotropic modifications—amorphous carbon and the two crystalline forms known as diamond and graphite. Amorphous carbon includes numerous common products such as wood charcoal, bone black, coke, lamp black, and carbon black. Each of these varieties of crystalline and amorphous carbon possesses properties that render it useful for a variety of purposes. 

The variety in properties of different produced carbon materials is conditioned by the electronic structure of a carbon atom. The redistribution of electron density, the formation of electronic clouds of different modifications around the atoms, the hybridization of orbitals (sp3-, sp2-, sp- hybridization) are responsible for the existence of different crystalline allotropic phases and their modifications. 

Solid selenium likewise exists m several allotropic forms. The i crystalline variety is labile, and may possibly occur in two modification both of which arc monoclmic.2 The grey crystalline form appears consist of two varieties, SeA and ScD, in dynamic equilibrium with ea other.1 Solid tellurium manifests allotropy, but to a much less pi nounccd degree. 

There is another fact which shows us that the law of Dulong and Petit cannot be strictly true. Many elements are capable of existing in several modifications which are distinguished from one another by their crystalline form, by their specific gravity, and by many other physical properties. These allotropic modifications have not the same specific heat, so that the atomic heat is not even constant for one and the same element. Still less, then, can we expect it to be constant for all elements. Wigand has shown that the denser modification of an element has always the smaller specific heat, as may be seen in the next table. 

A homogeneous chemical substance, for example an element such as sulphur or a compound such as water, may exist in several, and must be capable of existing in at least three different forms, viz. as gas, as liquid, and as solid. In many cases the solid may exist in various allotropic modifications, which differ from one another in crystalline form, melting point, density, specific heat, and, in fact, in all their ph5rsical properties. Every portion of matter which is in itself homogeneous, i.e. in which the smallest visible particles are exactly alike, and which is therefore separated in space from every other homogeneous but dissimilar portion of matter, was called by Willard Gibbs a phase. ... 

IR-3.4.1 Name of an element of indefinite molecular formula or structure IR-3.4.2 Allotropes (allotropic modifications) of elements IR-3.4.3 Names of allotropes of definite molecular formula IR-3.4.4 Crystalline allotropic modifications of an element... 

Crystalline allotropic modifications are polymorphs of the elements. Each can be named by adding the Pearson symbol (see Section IR-11.5.2)7 in parentheses after the name of the... 

When white phosphorus is heated at 200° under a pressure of 12,000 kgm. per sq. cm., transformation takes place into another allotropic modification known as black phosphorus. This forms a black crystalline solid, insoluble in carbon disulphide. It can be ignited with difficulty with a match, its ignition temperature in air being about 400°. When heated in a closed tube it vaporises and condenses to violet and white phosphorus. It differs from the other forms of phosphorus in being a conductor of electricity. Its density is 2 691, The question of the relative stability of violet and black phosphorus has perhaps not yet been definitely settled but the results obtained point to violet phosphorus being the more stable form, ... 

Some elements exist in more than one form. Familiar examples include (1) oxygen, found as O2 molecules, and ozone, found as O3 molecules, and (2) two crystalline forms of carbon—diamond and graphite (Figure 13-33). Different forms of the same element in the same physical state are called allotropic modifications, or allotropes. 

The form of elemental antimony that is stable at normal temperatures and pressures is the gray, metallic rhombohedral a-form, mp 630.7 °C, bp 1587 °C, density 6.70 gcm. Crystals are lustrous. They have a relatively high electrical resistivity (41.7 1 2cm at 20°C). The structure of o -8b consists of sheets of covalently bonded antimony stacked in layers, which are formed of puckered slx-membered rings. Each antimony forms three shorter bonds (2.91 A) in the same layer as well as three longer bonds (3.36 A) to antimony atoms in the adjacent layer. In addition to the a-form, other allotropes include a very unstable yellow form and black forms obtained electrolytically or by condensing the vapor on cold surfaces. Two crystalline allotropes are made by high-pressure techniques. At 85 kbar, a modification with a primitive cubic lattice is formed where each antimony atom is in an octahedral environment of six equidistant (2.96 A) neighbors. Further... 

---